Many conventional amplifiers including operational amplifiers employ an output buffer amplifier which is driven by a current mirror circuit. The current mirror circuit is in turn driven by a pair of differential current signals, usually obtained from a voltage to current differential amplifier. A gain control/frequency compensation device, for example a capacitor, a resistor, or a network including capacitors, resistors, and other elements, is connected to the output of the current mirror circuit for shaping the response with respect to both amplitude and frequency characteristics of the amplifier.
In order to improve the gain factor of such operational amplifiers to a theoretically infinite gain, a gain control device is used which includes a bias circuit that biases the third common terminal of the current mirror circuit and includes a voltage follower or a similar device which responds to the voltage at the output of the current mirror circuit to vary the bias on the common terminal of the current mirror and thereby drive the voltage on the input terminal of the current mirror circuit to track that on the output terminal of the current mirror circuit. As a result, the amplifier provides a large gain, virtually infinite, as it attempts to drive the voltage on the current mirror input terminal to track that on the output terminal, and null the difference in the differential input currents While this improved operational amplifier with the floating current mirror circuit does provide a much increased gain, it still suffers from the problem of distortion introduced in the output buffer amplifier. In normal use an external passive network is used in conjunction with the operational amplifier completing what is known as a feedback loop. This distortion thusly occurs internal to the feedback loop and distorts the overall performance. One technique that has been used to deal with such distortion in the past is to increase the quiescent power provided to the output amplifier stage, in order to minimize the distortion generated, but this increases the power consumption without increasing the power output and sacrifices the efficiency of the amplifier.